The present invention concerns methods and apparatuses for automatically recognizing photographic originals, e.g., negatives in a long strip of photographic negatives, which are unsuitable for copying. In particular, the present invention relates to methods and apparatuses of that type in which the original is scanned or measured with respect to density on a zonewise basis.
It is well known, in connection with automatic photographic printers and prereaders, to take measures intended to prevent markedly under- or overexposed originals, e.g., negatives, from being copied or printed.
Merely by way of example, U.S. Pat. No. 3,690,765 discloses a photographic printing or copying machine in which the average transparency of a sizable fraction of the surface area of each individual original is photoelectrically measured. The resultant measurement signal is then compared against an upper and a lower limit value, and copying occurs only if the measurement signal falls in the range intermediate the two limit values.
Federal Republic of Germany published patent application DT-OS No. 22 17 680 discloses a system in which the transparencies of a number of predetermined regions or zones of the original are individually measured. Copying is automatically prevented if all of these plural measurement signals fall below a certain lower limit value, or if all of the measurement signals exceed a certain upper limit value. I.e., copying is automatically prevented if the negative being measured is underexposed all over its surface or overexposed all over its surface. If even one of the plural measurement signals falls within the range intermediate the upper and lower limit values, copying is automatically commanded.
Automatic evaluating techniques of the types just mentioned are fraught with ambiguity. Firstly, with such techniques, it is extremely difficult to discriminate between, on the one hand, a negative which is underexposed and, on the other hand, a negative which is actually suitable for printing but exhibits low image contrast. In particular, the selection of the aforementioned lower limit value determines and limits the possibility of making such a discrimination, but the selection is made difficult, for example, by the fact that the subject filmed, i.e., as opposed to its background, may occupy only a relatively small fraction of the surface area of the negative. Secondly, the density of entirely unexposed film may exhibit considerable variation, depending for example upon manufacturer, aging and the film-development technique employed, so that often the fog density of an unexposed negative in a particular film strip can be higher than the average for the many film strips processed. Research has established that with known techniques such as described above the percentage of negatives unsuitable for printing but nevertheless printed, and likewise the percentage of negatives suitable for printing but nevertheless not printed, is unacceptably high. In particular, there is excessive printing of negatives on which adhesive stickers, or the like, have been applied, such negatives bearing so-called meaningless motifs and obviously being unsuitable for printing.